Chemical changes during anaerobic decomposition of hardwood, softwood, and old newsprint under mesophilic and thermophilic conditions.

The anaerobic decomposition of plant biomass is an important aspect of global organic carbon cycling. While the anaerobic metabolism of cellulose and hemicelluloses to methane and carbon dioxide are well-understood, evidence for the initial stages of lignin decomposition is fragmentary. The objective of this study was to look for evidence of chemical transformations of lignin in woody tissues [hardwood (HW), softwood (SW), and old newsprint (ONP)] after anaerobic decomposition using Klason and acid-soluble lignin, CuO oxidation, and 2D NMR. Tests were conducted under mesophilic and thermophilic conditions, and lignin associations with structural carbohydrates are retained. For HW and ONP, the carbon losses could be attributed to cellulose and hemicelluloses, while carbon loss in SW was attributable to an uncharacterized fraction (e.g., extractives etc.). The 2D NMR and chemical degradation methods revealed slight reductions in β-O-4 linkages for HW and ONP, with no depolymerization of lignin in any substrate.